1. Field of the Invention
The present invention relates to a retrofocus-type wide angle lens and a camera including the retrofocus-type wide angle lens. In particular, the present invention relates to a retrofocus-type wide angle lens having long back focus, which is appropriate for use for example in a medium-size single-lens reflex camera, and a camera including the retrofocus-type wide angle lens.
2. Description of the Related Art
Generally, a wide angle lens for a single-lens reflex camera needs to have a sufficient back focus. Therefore, many wide angle lenses for single-lens reflex cameras adopt retrofocus-type lens structure, in which a negative lens group and a positive lens group are arranged in this order from the object side and the front side and the rear side of the lens system are asymmetric with respect to a stop. Such retrofocus-type wide angle lenses are disclosed, for example, in Japanese Unexamined Patent Publication No. 2002-214526 (Patent Document 1), U.S. Pat. No. 6,697,202 (Patent Document 2), and U.S. Pat. No. 6,894,847 (Patent Document 3).
The retrofocus-type lens is appropriate to secure a long back focus. However, since the retrofocus-type lens is asymmetric with respect to the stop as described above, it is difficult to correct aberrations related to angles of view, especially, lateral chromatic aberrations. Meanwhile, digital cameras became widely used in recent years. When a digital back or the like is mounted on a camera to replace a film, more excellent correction of lateral chromatic aberrations is required because of the light receiving characteristics of the digital back or the like. In the lenses disclosed in Patent Documents 1 through 3, correction of lateral chromatic aberrations is not sufficient. For example, in Patent Document 1, it appears that lateral chromatic aberrations are corrected by using two lenses made of special low-dispersion glass in a rear lens group. However, correction of the aberrations is insufficient.